The present invention relates to a plug connection for connecting two pipe sections of a drill pipe and a corresponding pipe section
The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention.
When drilling in the soil with drill pipes, in particular for producing so-called horizontal boreholes which extend essentially parallel to the surface or have a relatively small slope angle with respect to the surface, a drill head is advanced by a drill pipe by a drive system arranged on the surface or in an excavation pit. The employed pipes consist of individual interconnected pipe sections which are sequentially added and connected to the rear end of the already installed pipe—commensurate with the drill path. Typically, threaded connections are used.
Boreholes are typically formed in soft soil through radial displacement and compaction of the soil. The drill head is therefore generally rotated only with a low rotation speed, if rotated at all, in addition to the static advance.
Conversely, for drilling through rock, i.e. drilling in rock or rocky soil formations, the rock needs to be crushed due to its insufficient deformability and to remove the crushed rock from the already produced borehole. Crushing the rock requires relatively high rotation speeds of the drill head which would cause considerable wear of the drill pipe, if the drill pipe would rotate in the borehole with the same rotation speed.
Essentially two different designs of rock drill systems, which operate without rapid rotation of the drill pipe that is in contact with the wall of the borehole, have come to dominate the market.
A first of these designs is based of the use of an in-hole motor which rotatably drives the drill head directly and not by way of the drill pipe. Instead, the assembly composed of the drill head and the in-hole motor is affixed on the front end of the drill pipe, with the drill pipe generating the axial pressure required for advancing the borehole. Because the rotation of the drill head required for creating the borehole is generated by the in-hole motor, the drill pipe itself in these drilling systems need not be rotatably driven. The wear of the drill pipe is therefore relatively small. So-called “mud-motors” are generally used as in-hole motors, with a drive fluid being routed through a turbine under high pressure to produce the rotation. This drive fluid is usually a drilling fluid which, after flowing through the mud motor, exits the borehole through outlet openings in the region of the drill head, in order to cool and lubricate the drill head and in order to wash out the removed cuttings through the annular space between the borehole wall and the drill pipe. Disadvantageously, these rock drilling systems based on in-hole motors not only consume a large quantity of drilling fluid, but also have low efficiency (e.g. 800 Nm when consuming 320 L/min of washing fluid).
The second commonly used design of rock drilling systems is based on using a double drill pipe. In these drilling systems, the drill head is additionally rotatably driven with the drive system arranged at the surface or in an excavation pit via an inner pipe of the double drill pipe. The drive system also advances the drill head. The inner pipe is hereby rotatably supported within an outer pipe of the double drill pipe. The outer pipe then does not rotate at all or rotates only at a low rotation speed. This type of rock drilling system also limits wear of the drill pipe, because the outer pipe contacting the rocky wall of the borehole does not rotate at all or only at a low rotation speed, whereas the inner pipe which is driven at a high rotation speed can be supported in the outer pipe to reduce wear.
In almost all conventional rock drilling systems employing double drill pipes, the individual pipe sections of the outer pipe and of the inner pipe are screwed together. This is, on one hand, quite time-consuming. On the other hand, large torques are transmitted at least via the inner pipe, causing severe “jamming” of the threaded connections (which operate in a self-locking fashion), so that the individual threaded connections need to be detached mechanically after the borehole is completed. This significantly increases the structural complexity of such rock drilling systems—in addition to the complex structure of the double drill pipe itself.
To reduce this time-consuming operation and to simplify the structure, a drilling system with a double drill pipe has been developed (see EP 0 817 901 B1), wherein the individual pipe sections of the inner pipe are no longer connected with each other by a screw connection, but are instead connected by a simple axial plug connection. For this purpose, each of the inner pipe sections has at one of its ends a connecting pin with a hexagonal cross-section and at the corresponding other end a matching connecting bushing, so that the two pipe sections can be readily connected with one another by inserting the connecting pin of one pipe section into the connecting bushing of the other pipe section. The axial plug connection allows a particularly simple and rapid connection of the individual pipe sections of the inner pipe. Disadvantageously, the two pipe sections must be exactly (rotationally) aligned before the two connecting parts can be plugged together, so as to overlap the two cross-sectional contours of the connecting pin and of the connecting bushing.
It would therefore be desirable and advantageous to obviate prior art shortcomings and to provide an improved plug connection for a drill pipe and in particular for the inner pipe of a double drill pipe.